Restorable fe-cr-ni alloy



May 28, 1957 A. R. WAGNER RESTORABLE: F@ cr-Nr ALLOY I5 Sheets-Sheet 1 Filed Aug. 29, 1955 May 28, 1957 Filed Aug. 29, 1'955 A. R; WAGNER 2,793,948

REsToRABLE F@ cr-NLD ALLOY 3 Sheets-Sheet .2

limited States Patent This is a continuation-impart of my co-pending patent application Ser. No. 357,250, tiled May 25, 1953, now Patent No. 2,763,543.

In modern techni-c there is a continuously increasing demand for heat resistant materials, .and particularly for heat resistant steels. The demand for high heat resistant properties of heat resistant steels, and particularly for low sensitivity to overheating, has also increased due to the rapid development of gas turbines. A number of alloy compositions have been suggested for the purpose of attaining better properties at higher temperatures. At a relatively early date it was found and contirmed, that austenitic steel alloys with suiciently high contents of chromium and nickel and certain other elements such as tungsten and cobalt have excellent heat resistant properties at tempera-tures around and somewhat above 500 C. For the purpose of counteracting precipitation of carbides at the grain boundaries it has also been suggested to add such carbide forming elements as columbium, titanium, tantalum and vanadium. However, it very soon turned out particularly due to the rapi-d development of the aviation technic during the Second World War that the austenitic heat resistant C'rNi-steels of the older types were not up to requirements' when subjected to high temperature for longer periods. Then the search `for new materials was started, and Vthereby inter alia by researches in the Mond Nickel Company the modern high alloyed materials were invented on the basis of nickel and chromium with a basic composition of about 80% Ni and 20% Cr. VThese alloys further shall contain 0.05-0.5% C. and one or more carbide forming elements among which titanium has been given preference.

Important progress has been obtained with heat resistant chromium-nickel steels as Well as with special alloys on chromium-nickel basis in that said alloys Ihave been given such a composition that they can be subjected to a heat treatment causing precipitation hardening.

In alloys ha-rdenable by precipitation which below will be related to as ordinary aging as is known there is after a common quenching from a high dissolving ternperature obtaineda supersaturated solid solution of certain alloying elements in the basic'rnass of the alloy. On -re'heating to a temperature considerably below the above mentioned dissolving temperature the substances which were present in solid solution will be precipitated in a phase of very ne destribution giving the alloy Icertain improved properties such las hardness and increased heat strength. However, if for an ordinary aging material a certain temperature which is characteristic for each composition is surpassed the precipitated particles become coarser and the properties become considerably deteriorated (so called over aging). This disadvantage is maintained even when the temperature is restored to a degree below the said characteristic temperature. In Aorder to arrive again at the most advantageous properties such a precipitation hardening alloy must be subjected to a complete new heat treatment for dissolving and renewed precipitating of the precipitable constituents. Alloys having the ability of restoration have a much higher resistancy against exhaustion and vibration stresses than precipitation hardening alloys due to the absence of precipitated stable phases.

`From aluminium alloys and especially Duralumin it is known that fthe aging at room temperature which is characteristic for these alloys is due to the precipitation of a metastable phase and that this p hase can be dissolved by heating to a moderate temperature considerably below the dissolving temperature of stable equilibriumvand that upon `a succeeding cooling to room temperature the said aging process after 4some time occurs again. This phenomenon which can be repeated any number of times is generally called restoration and `alloys having such properties Vwill below be related to as restorable alloys in contradistinction to the ordinary aging alloys which can not be restored.

During his yresearch on heat-resistant and lireproof steel alloys the inventor made some discoveries which seemed -to hint at the possibility that restoration can be achieved in steel alloys of a suitable composition and at considerably higher temperature than in the `case of Duralumin. Through a systematic investigation of heat resistant steels with varying compositions and mainly austenitic structure it has now been veried that the observations were correct and that some o-f these alloys have the property yof being restorable. One condition for this effect is that the alloy contains titanium. A

further condition is that the titanium content is in a certain relation to the y.sum of the contents of iron and cobalt on one hand and to the contents of carbon and nitrogen on the other hand. This relation is best seen from the accompanying Fig. 1 diagrammatically showing the lowest required content of active titanium as Va function of the sum of the iron and cobalt contents. Active titanium hereby means that titanium content whichis not combined with carbon and nitrogen. The titanium content `combined can be calculated with lsutlicient accuracy by multiplying the sum of the nitrogen `and carbon contents by four.

Based upon the above related discoveries the present invention in the tirst line involves objects which are intended duringutheir use to be subjected to temperatures of at least 650 C. and`which are insensible for accidental superheatings above the temperature at which said objects are intended to work thereby that their resistancy and hardness are restored when returning from superheating to the normal working temperature. These objects are substantially characterized in that they are manufactures from a non-precipitation hardenable steel alloy having at least predominating austeniti structure and containing at least one gamma forming element, taken from the group consisting of nickel and manganese, at least 20% of iron-l-cobalt, at least 13.7% Cr, maximum 0.6% CQ, and nitrogen in contents usual in steel alloys, said alloy further containing titanium in such contents that the active titanium content depending upon the sum content of iron Iand cobalt is at least as high as the content dened by a curve in a coordinate system passing through the points k20% Fe-l-Co/ 0.25 Ti; 30% Fe-l-Co/0.35% Ti; 40% lFe-l-Co/O.5% Ti; 50% 'Fe-{.Co/0.7% Ti; `60% lFe-l-Co/0.9% Ti; 70% Fe-{-Co/1.25% Ti; Fe-lCo/l.5% Ti.

In the following the invention is to be described more in detail with reference to the accompanying diagrammatical drawings.

Fig. i illustrates the lower limit for the active titanium content as a function of the sum of the iron and cobalt contents. Fig. 2 illustra-tes the lack of restoration in steels of the type hitherto used in that the hardness is decreased by superheating and is maintained at the lower value when cooling again to working temperature (in this case 700 C.).

Fig. 3 illustrates the eiect of restoration in alloys according to the invention in that the hardness decreased jcontent becomes correspondingly higher.

3 due to superheating above the working temperature (700 C.) is restored when returning to said working temperature. Fig. 4 finally illustrates the behavior of restoration at repeated superheatings. l All points of the diagrams have been established by practical trials. j

The curve in Fig. 1 gives the total titanium content for alloys Vwhich are practically free from carbon and nitrogen. When carbon and nitrogen are present the value of the titanium -content taken from the curve must be increased, advantageously by about 4 times the sum of the carbon and the nitrogen contents. Said increase of the vtitanium content is due to the fact, that titanium is bonded ,asf titanium carbide and titanium nitride wherefore the active titanium content at disposal for creating the lfelect `of restoration becomes lower than the total content. If other carbide forming elements such as vanadium, tantalum and columbium are present these also will be bonded to carbon whereby the active titanium The active titanium content therefore could be defined as that part of the total titanium content which is in solid solution in the alloy.

Sometimes alloys containing the minimum contents of titanium defined by the curve of Fig. 1 have the ability .of restoration only partially developed. In order to have said ability with certainty fully developed the titanium content should be somewhat higher than the values delined by thel curve. With regard to the desirability that the alloys Vshall be easily workable at high temperatures ,the titanium content on the other hand should, as a rule, ynot be'kept too high. The most advantageous active titanium contents therefore are between the curve of Fig. 1 and about 2 percent.

In the following four examples are given of different basic alloys which possess the properties aimed at with Vthe invention.

1. Cr-28%, Ni 8-25%, active Ti 0.65-1.35%. Co `can advantageously be added in contents of between l- 2. Cr -l8%, Ni 1li-28%, active Ti 0.8-1.40%. ',Co can advantageously be added in contents of between 3. Cr -30%, Ni B2-45%, active Ti 0.4-0.75%. YCo Acan advantageously be added in contents `of between .l0-25%,

74. Cr l0-30% and preferably 18-30%, Ni 10-45%, preferably -28%, at least one of the elements MO, W and V each in a content of up to 1%, active Ti 0.35- 1.68%. Co can advantageously be added in contents of between 10-25 Also in alloys of Examples 1-3 above at least one of the elements Mo, W and V can be added in contents of below 1% each.

In all alloys according to the above examples the rest is iron with its common accessory elements such as C, Si, Mn, N, S and P. When the titanium content is near the lower limit it is, as a rule, suitable to have a silicon content of about 0.5-1.0%.V The alloys further may contain such additional elements which favorably inuence such properties as strength and malleability at high temperatures as well as at room temperature. Such additions may be one or more of the alloying elements Si, Al, W, Mo, V, Nb, Ta, Cu, Th, Mg, Zr and low contents of rare earth metals, alkaline earth metals, beryllium, boron and uranium as well as phosphorus arsenic, antimony, nitrogen and sulfur.

On trials it has been settled that the above enumerated elements in such contents which'are usuallyr'equired do not deteriorate the restoration elect. A' certain low aluminium content is practically always obtained from the available titanium alloys used as alloying element in the production of the alloys. t

The invention also involves a method of producing restorable objects. This method comprises forming the objects at a temperature above 900 C. and suitably above 1000 C., then bringing them to accept a temperature at which they are intended'to be used or the temperature giving the highest heat strength; vThis alteration of the temperature of the objects can be performed either by cooling from the temperature at which thel objects haveV been formed, possibly with a stall in the temperature range between 850 and 980 C. or by cooling from the temperature at which the objects are formed down to room temperature and reheating to the temperature at which the objects are intended to be used or to that temperature which gives* the highest heat strength. Also inV Athis Alast mentioned case itA can be advantageous for a short period of time to keep the temperature between .S50-980- C., sincethereby theftime of heating can be considerably decreased. The cooling can be performed Ain a furnace, inair, in oil or water, and shall, as a rule,

be madejslowly,`even if quenching in a liquid is sometimes possible. Generally it is most advantageous to perform the cooling stepwise in a furnace.

Heating in the temperature range S50-980 C.-the exact temperature being chosen according to the composition of the alloy-is not absolutely necessary but it has been found that thereby the necessary time -for which the alloy must be kept at its temperature of working or at that temperature which after cooling gives the highest heat strength can be considerably decreased.

That heating at the working temperature which is required .in order to get thevdesired properties completely developed can be performedY during the utilization of the alloy.

In the following there is made a comparison between a normally aging alloy'and a restorable alloy and, further, it is shown how the hardness is inuenced by vari-- .ations in the temperature. Then in Table IV there are given a number of alloys which have been treated according to this invention. n

Table I and Fig. y2 illustrate how the hardness is varied when heat treating cylindrical samples of 22 mm. diameter and 10 mm. height from a normally aging alloy with the composition: C=0.22'%, Cr= 14.7%, Ni=24.6% and Al=3.5%, which samples first were heated at 1050- 1l00 C. for l hour and then cooled in air. Each of the samples were then heated at a fixed temperature within the range 700-950 C. for a period of l5 hours. After the first heating and cooling to room temperature the hardness was measured. Then the samples were subjected to the second heating, cooled again to room ternperature and then the hardness again measured. From the table and the curve of Fig. 2 it is seen how the hardness decreases at higher temperatures. The left hand part of the tigure shows the hardness values after the first heating and the right hand part the values after the second heating. The arrows illustrate the directions in which the resistance values are altered from which it is seen (just as from Table I below) that the hardness values obtained after the superheating are maintained even after reheating at 700 C. for 15 hours.

Tble I First heating second heating Table II and Fig. 3 illustratev in a corresponding manner the course when using a restorable lalloy according to definite conclusions regarding hardness arid resistancy at high temperatures. However, with the austenitit':y alloys here in question a variation in hardness at room tem# perature can be utilized as a criterion of the variation points and arrows inserted in Fig. 3. 5 of the heat strength in the same direction. Practical trials have proved that this theory is correct. Thus, if a mate- Table Il rial has been superheated so that its hardness at room temperature is decreased, its heat strength was simultaneously decreased, and when such material then due to First heating second nesting 10 restoration recovers its earlier hardness (at 'room temperature) this shows that it has also recovered its earlier Tgmp., HB Tsmp., HB heat strength.

C- C- In Table IV there are given examples of a number of steel alloys within the scope of this invention and also a 32,8 38 ggg 15 number of alloys outside of but near the limits of the 800 290 700 355 invention. In the last mentioned group there are given ggg ggg ggg ggg some of the commonly utilized heat resistant steel alloys. 950 190 700 360 Sometimes, and particularly in the cases of more complex alloys, it is advantageous after hot working the ob- 20 jects to reheat them to at least 1000* C. and suitably to about 1100u C. as stated for the samples according to ItndTlble ,1 mi corrSpOnciugf-Flg' 4 thirethls mus" Table I and Fig. 2 in order thereby to complete the dif- I e 0W e tar ,SESS .1S vage Oitlntl an e sfne fusion or dissolving of the alloying elements and thereby oi lag-:yor 105,7 Slilezn 70,11 W21 T.1c7(;,nposltl.on to equalize the structure. Such reheating is particularly T21 1,70 ET o id th 0 au 1 1 im v 25 advantageous in combination with a stall inthe tempert 1 0.) rv et qnetfm t e sauge SamRteh-1s tsl? )ec e ature range S50-980 C. during the cooling. uggswy aAtma mgh mpra uest ltu e ringe Further investigations have shown that the upper limit t-d. gbl HI erdeacd rte; Intel; dat. e tlmpera me for the active titanium content depends on the silicon .Sta e ful i ai ug er efs a e dinges e rtneasur' content in such a Way that said upper limit coutent i11- mg o t; ar tllss. Eis een fr. ,mle ha droom dempir' 30 creases when the silicon content decreases below 0.6%. ature' h rtom 1st 1t 1S c ear 0W e ar ness epe s Thus, at a silicon content of at least 0.6% the upper upon t e empara me' limit content for the active titanium is about 2.0%, at a silicon content of 0.3% said upper limit content is about Table III 2.3%, and at a silicon content of 0.1% said upper limit ."-5 content is about 2.6%. The upper limit for the active Tempuo C. Timm En titanium content at a silicon content below 0.6% may hours therefore be represented in a Ti-Si-coor'dinate system by a curve passing through said three points, as illustrated in E lg Fig. 5. 10 220 40 I have also found, that in alloys according to the invention, containing one or both of the elements nickel a 161 and cobalt, a manganese content of above Ca 0.3% de- 7 240 creases the heat strength of the alloy. If, therefore, a high heat strength is desired, the manganese content l 45 should not exceed about 0.3%, and should preferably be In the examples there are given the values of hardabout O15-0.20%. This is important when nickel or ness measured at room temperature. Normally it is not cobalt, or both of them, are present as austenite forming possible from the hardness at room temperature to draw elements'.

Table IV Ti Alloy C Cr Nt Mo W V Co Nb/Ta Be Restorable total active 0. 038 15. 4 45. 7 0. 6 0.038 15.4 46.4 0.9 0.14 19.6 20.6 0.09 20.1 18.8 1.0 0.08 19.2 18.8 2.1 0.14` 14. 9 25. 7 1. 0 0. 00 14.7 24.1 0.8 0.15 15.2 20.0 1.7 0.21 14.0 24.4 2.1 0. 047 15. 0 24. 9 2. 1 0.08 14.4 23.6 2.2 0.19 18.7 20.1 1.1 0.18 18.4 18. 2.6 0.20 17.7 19.1 2.7 0. 032 1s. 7 20.1 0. 8 0. 030 15. 9 15. 2 0. 94 0. 00s 15.9 15. 2 1. s 0.12 25.4 23.4 0.30 0. 10 24. 7 19. 9 0. s2 0. 15 25. 0 12. 8 0. 49 0. 09 17. 7 11. e 0. 27 0.13 19.9 9.5 0.33 0.05 18.0 8.7 0.48

Hummm-The alloys contain silicon and manganese in contents up to 2% and further the usual accessory elements in steel alloys.

and aluminum up to 0.5%

V1. `Objects which during their use are to be subjected to temperatures of at least 650 C. and which are nonsensible to superheating above their temperature of utilization by recovering their strength and hardness when the temperature is returned tosaid temperature of utilization, characterized in that they are produced from a nonprecipitation hardening alloy of at least predominantingl'y austenitic structure and containing at least one` of the gamma forming elements Niv and Mn Within the range from 10% to 45%,' from a trace up to 1% of at least one of the elements V, Mo and W, at least 20% Fe-l-Co, not more than 0.6% C, Cr within the range from 13.7% to 30%, the common accessory elements Si, N,` P and S andrfurther Tiin such content that the active Ti-content is at least the one defined by a curve in a coordinate system set forth in Fig. 1 of the drawings, said curve passing through the points 20% Fe-[Co and 0.25% Ti; 30% Fev-l-Co and 0.35% Ti; 40 Fed-Co and 0.5% Ti; 50% Fe-i-Co and 0.7% Ti; 60% Fe-l-Co and 0.9% Ti; 70% Fe-l-Co and 1.25% Ti; 75% Fe+Co and 1.5% Ti, the upper limit for the active titanium content being below 2% when the silicon content is above 0.6% and beingdefined by a curve in a Ti-Si coordinate system set forth in Fig. 5 of the drawings, said curve passing through the points 0.6% Si and 2.0% Ti; 0.3% Si and 2.3% Ti; 0.1% Si and 2.6% .Ti when the silicon content is below 0.6%..

2. Objects as claimed in'claim 11, containing at least one of the elements nickel and cobalt, in which t-he manganese content is below 0.3%

3. Objectives according to claim 1, in which at Ti contents c-lose to the-Values detined by the curvethe Si' con# tent is'between 0.5 and 1.0%

4. Objects according to cla-im] containing up to 5% of at least one of the elements Ta, Nb, Al, C-u, Mg, Zr, and low percentages of at least one of the elements Th, Ce, Be, 1B, U, P, As, Sb, S, and :alkaline earth metals.

5. A method of heat treating resistant objects of an at least predominantly austenit-ic alloy containing at least one of the gamma yforming elements Ni and Mn within the range trom to 45%, -at least 20% Fe-l-Co, from a trace up t-o 1% of at least one of the elements V, Mo and W, not more than 0.6% C, :Cr .within the range from 13.7% to 30%, the common accessory elements Si, N, P and S and further Ti in such content that'the active Ti# content is at least the one dened by a curve in a coordinate system set Iforth in Fig. 1 'of the drawings, said curve passing through the point Fe-i-Co -and 0.25% Ti; 30% \Fel-Co and 0.35% Ti; 40% Fe-l-.C-o and 0.5% Ti; 50% y.lB'e-l-C-o Iand 0.7% Ti; 60% -Fe-i-Co and 0.9% Ti; 70% Fe-l-Co and 1.25% Ti; 75% Fe+Co and 1.5% Ti,

the upper limit'forv the `active titanium content'being be-l ing below 2% when the silicon content is above 0.6% and 4being defined by a curve -in a T-i-Sicoordinate system set lforth in Fig.' 5 of lthe drawings, said curve passing through the points 0.6% Si and A2.0% Ti; 0.3% Si and 2.3% Ti; 0.1% Si and 2.6% T-i when the silicon content is below 0.6%, which heat treatment comprises hot Working at 900 C. at least and preferably at a temperature above v1000 C. -with subsequent restoring the temperature at a value at which the objects are to be utilized yor at -which the best heat resistant properties are obtained.

6. A method as claimed in claim 5, 'in which the temperature is Irestored by cooling directly to said tempera t-ure of utiliza-tion. l

7. A method as claimed in claim 5, in which the temperature is restored by cooling with a lstallin the temperature range between 850 and 980 C.l

8. A method as claimed in claim 5,'-in which after hot working the object is cooled down t0 room temperature and then reheated to the temperature at which the object is intended to be utilized or to the temperature giving. the highest heat resistancy. l 9. A method as claimed in claim 5, in which the object after hot working is cooled down to room temperature, then reheated within the temperature range 950-980" C. and finally cooled to the temperature at Which the object is intended to be utilized or to the temperature giving the best Iheat resistant'properties. 10. A method as claimed in claim 8, characterized in that the heating to the tempreatu-re at which `the object is intended to be used is rst performed -duringthe utilization of the object.

11. A method las claimedin claim 9, characterized in that the heating 4to the temperature at which .the object-is' intended to be used is `first performed dur-ing the utilization `of the object.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Metal Progress, October 1950, page 505. 

1. OBJECTS WHICH DURING THEIR USE ARE TO BE SUBJECTED TO TEMPERATURES OF AT LEAST 650*C. AND WHICH ARE NONSENSIBLE TO SUPERHEATING ABOVE THEIR TEMPERATURE OF UTILZATION BY RECOVERING THEIR STRENGTH AND HARDNESS WHEN THE TEMPERATURE IS RETURNED TO SAID TEMPERATURE OF UTILIZATION, CHARACTERIZED IN THAT THEY ARE PRODUCED FROM A NONPRECIPITATION HARDENING ALLOY OF AT LEAST PREDOMINANTINGLY AUSTENITIC STRUCTURE AND CONTAINING AT LEAST ONE OF THE GAMMA FORMING ELEMENTS NI AND MN WITHIN THE RANGE FROM 10% TO 45%, FROM A TRACE UP TO 1% OF AT LEAST ONE OF THE ELEMENTS V, MO AND W, AT LEAST 20% FE+CO, NOT MORE THAN 0.6% C, CR WITHIN THE RANGE FROM 13.7% TO 30%, THE COMMON ACCESSORY ELEMENTS SI, N, P AND S AND FURTHER TI IN SUCH CONTENT THAT THE ACTIVE TI-CONTENT IS AT LEAST THE ONE DEFINED BY A CURVE IN A COORDINATE SYSTEM SET FORTH IN FIG. 1 OF THE DRAWINGS, SAID CURVE PASSING THROUGH THE POINTS 20% FE+CO AND 0.25% TI; 30% FE+CO AND 0.35% TI; 40 FE+CO AND 0.5% TI; 50% FE+CO AND 0.7% TI; 60% FE+CO AND 0.9% TI; 70% FE+CO AND 1.25% TI; 75% FE+CO AND 1.5% TI, THE UPPER LIMIT FOR THE ACTIVE TITANIUM CONTENT BEING BELOW 2% WHEN THE SILICON CONTENT IS ABOVE 0.6% AND BEING DEFINED BY A CURVE IN A TI;-SI COORDINATE SYSTEM SET FORTH IN FIG. 5 OF THE DRAWINGS, SAID CURVE PASSING THROUGH THE POINTS 0.6% SI AND 2.0% TI; 0.3% SI AND 2.3% TI;
 0. 1% SI AND 2.6% TI WHEN THE SILCON CONTENT IS BELOW 0.6% 